1. Field of the Invention
The present invention relates to an optical head in use with an optical recording/reproducing apparatus, and more particularly to an optical head which is used in an optical filing system and the like and records information optically on a recording medium and reads out the recorded information therefrom.
2. Description of the Related Art
Such an optical filing apparatus is known as an optomagnetic disk apparatus in which information is recorded on an optical disk and reproduced and erased therefrom. In this apparatus, information is recorded and erased by a radiating laser beam on the recording surface of the optical disk and magnetizing the same. When only a laser beam is radiated on the optical disk on which information has been already recorded, the information is reproduced.
The recording erasure reproduction of information are performed by use of an optical head. This optical head is provided with a laser diode for generating a laser beam, and a beam splitter for irradiating the optical disk with the laser beam and guiding the laser beam reflected by the optical disk to an optical deflector.
In addition to these optical elements, the optical head is further provided with: an ellipse-correcting prism which shapes the cross section of a laser beam to be substantially circular and which causes the laser beam to have substantially the same intensity distribution between the horizontal and vertical directions of a cross section of the laser beam; a 1/2 wavelength plate which rotates the plane of polarization of the laser beam reflected by the optical disk; and a beam splitter which splits the laser beam whose plane of polarization has been rotated by the 1/2 wavelength plate, into a P-polarized light component and an S-polarized light component. It should be noted that the direction of polarization of a laser beam is parallel to a plane containing both the optical axis and the shorter axis of the cross section of an elliptic laser beam. This being so, the laser diode mentioned above is arranged to face the beam-incident surface of the ellipse-correcting prism, such that the direction of polarization of the laser beam can be either horizontal or vertical with respect to the beam-incident surface.
In the optical head, the laser diode generates a laser beam having an elliptic cross section, and the ellipse-correcting prism changes the cross section of the laser beam from elliptic to substantially circular or causes the laser beam to have a substantially the same intensity distribution between the horizontal and vertical directions of a cross section of the laser beam. Thereafter, the laser beam is focused on the information recording medium or the recording surface of the optical disk after passing through a polarized beam splitter and an objective lens and then reflected by the recording surface of the optical disk to the polarized beam splitter through the objective lens. The reflected laser beam is separated from the laser beams traveling to the optical disk and divided into a focus-and-track detecting laser beam and an information-reproducing laser beam.
The focus-and-track detecting laser beam is converted into focus control signals and track control signals and supplied to a lens moving mechanism for defining the position of the objective lens so as to accurately trace data recorded on the optical disk.
The polarization plane of the information-reproducing laser beam is rotated through 45.degree. by an 1/2 wavelength plate, and then, the rotated laser beam are lead to the polarized beam splitter to be divided into P-polarized light component passing through the beam splitter and S-polarized light component reflected thereby. After detected by elements of respective optical detectors, the P-component and the S-component are converted into electrical signals and reproduced as corresponding pieces of information recorded on the optical disk. The 1/2 wavelength plate is arranged such that the amount of P-component and that of S-component of a laser beam are equal to each other in the case where the optical disk has not yet been used or magnetized. In other words, the plane of polarization of the laser beam reflected by the optical disk is rotated approximately 45.degree. with reference to the beam-incident surface of the ellipse-correcting prism.
When information is recorded on the optical disk, the recording surface of the optical disk is initialized. By this initialization, the recording surface is magnetized in the same direction. At this time, the direction of polarization of the laser beam reflected by the optical disk is shifted such that the P-component becomes larger than that of S-component. This condition is similar to that of the case where data "0" is recorded. On the other hand, in the case where data "1" is recorded, the recording surface is magnetized in the opposite direction, and the direction of polarization of the laser beam reflected by the optical disk is shifted such that the S-component becomes larger than that of P-component. In this manner, the difference between the P-component and the S-component is detected and is reproduced as information recorded on the optical disk.
The intensity of the laser beam is modulated in response to information at the time of recording. At the same time when the laser beam is incident on the optical disk, a magnetic field is generated by a magnet arranged so as to face the optical head. Pits are formed on the optical disk by changing the directions of magnetization in the areas where the laser beam is irradiated such that information corresponding to the laser beam which is modulated in intensity is recorded on the optical disk. On the contrary, a laser beam having a predetermined intensity larger than that of information which has already recorded on the optical disk are projected thereon to erase the information. Since, in this case, a magnetic field is applied to the optical disk as is in the case of recording, the direction of the magnetization in the area illuminated with the laser beam is returned to the original state. At the reproduction of information, a laser beam is used which has a predetermined intensity smaller than that of the laser beam which is used for erasing the information. In this stage, when information is not recorded or no pits exit on the projected area, no rotation occurs in the polarization plane of the laser beam reflected by the optical disk. Whereas, when any information or any pit exists on the optical disk, the polarization plane of the reflected laser beam is rotated. The rotation of the polarization plane is detected by the above-mentioned detecting method and the information is reproduced.
In order to improve the reliability of a reproduced signal, it is desired that the optical head be provided with not only the 1/2 wavelength plate but also means for dividing the laser beam reflected by an unused or unmagnetized optical disk into P- and S-components substantially at the same ratio. Various additional mechanism such as for rotating the 1/2 wavelength plate about the optical axis of the laser beam. This structure creates the problem in which the number in optical parts of the optical head is increased, the optical head is rendered bulky and heavy and the durability of the optical head is deteriorated. Further, since the optical characteristics of the 1/2 wavelength plates themselves differ from plate to plate, assembly and adjustment of the optical head are made complicated, resulting in the increase of assembling cost.